Pennsylvania's Gov. Richard L. Thornburgh, a Republican, questioned the governors' expertise on defense matters. He said he did not understand how state leaders, ''without a Defense Department of our own,'' could know how big a budget was needed to protect the US.

At breakfast the other morning, one of the governors who criticized the President's defense spending, Illinois' GOP chief executive James R. Thompson, was asked: ''How, at a distance, can you know about cutting defense spending? What do you know about the defense budget?''

''Well, Governor Thornburgh has said we don't have any competency in the defense area,'' Governor Thompson replied. ''And he was answered by Governor Snelling (Richard A. Snelling, Republican governor of Vermont), who said that we are not trying to choose between the MX and the B-1 or any sort of missile systems.

''What we are saying is that at a time when the fundamental economy of the nation is just now emerging from the worst four-year battering it has had since the '30s, we clearly have to restrain growth in federal spending across the board if we are to prevent the economy from going back into a recession which would be even worse than the one we just had.

''While we have no ex-generals assigned to the NGA's staff to give us a defense competence, we can, we think, as political leaders, say that an acceptable range of spending growth (in the military) - somewhere in the 4 to 6 percent area - ought to be sufficient.

''And we are fortified in that conclusion by statements that have been made by members of Congress who presumably possess the competence to assess the proper range of defense spending. Also our position mirrors the position of experts in this field who do have defense competence.''

Obviously Thompson, vice-chairman of the NGA executive committee, would very much like to get the nation's governors involved in the national budget debate. Further, most of these state leaders want to get involved. And since these governors are constantly wrestling with their own very difficult money problems at the state level, they do qualify as experts on how to run a government's budget.